This invention relates to a method for heat stabilizing pre-packaged food products on a continuous basis.
More particularly, this invention is concerned with a continuous stabilization method for stabilizing, by exposure to microwaves, food products which have been packaged and sealed in containers at least partly transparent to microwaves.
In order to impart good preservability and prolonged shelf life to generic food products, and specifically to stuffed and unstuffed pasta products, the enzymes present in such products require to be deactivated, and the growth of microorganisms must be inhibited as by drastically reducing their content or even killing all of the microorganisms present therein.
A suitable heat treatment of the food products, as carried out at temperature levels and for time periods to be selected in accordance with the results sought, such as at 80.degree. C. for pasteurization and 120.degree. to 130.degree. C. for sterilization, is the most commonly used of conventional practices, and in many cases, is still the most reliable one for the purpose just outlined.
However, this prior technique has had since its inception a serious drawback tied to the very time/temperature parameters, which are known to affect significantly the organoleptic and nutritional properties of food products subjected to heat treatment.
This drawback is aggravated by the food products being packaged and sealed within containers where, to ensure that their parts at the container middle can attain a predetermined processing temperature, the peripheral parts, i.e. those closest to the container walls, are likely to exceed said temperature and remain under such a critical condition for a time period which may be significantly long.
Organoleptic and nutritional deterioration of the peripheral parts is the unavoidable consequence of the above.
A factual technical contribution toward obviating the above-noted drawback has come from the use of electromagnetic energy (microwaves), which enables food products to be heated at a very fast rate even to relatively high temperature levels, such as those required for sterilization, in the range of about 120.degree. to 130.degree. C.
The ability to shorten significantly the residence time of food products under high temperature conditions has been in many cases resolutive from the standpoint of safeguarding their organoleptic properties.
Resorting to microwaves also has, however, drawbacks which are not easily overcome or overcome at all and originate basically from an uneven application of heat to the product which is dependent, in turn and for a given time of exposure to the microwaves, on the very nature of microwaves, the temperature increase sought (the larger the temperature increase the more uneven the product heating), the chemical and physical characteristics of the product (e.g., the higher the moisture content the less uniform the heating), the geometrical characteristics of the product, taking into account its density and the progressive loss of energy undergone by the impinging microwaves.
Further aspects of the technical problem involved in the microwave processing of food products sealed within containers which are transparent to microwaves are the need to sanitize (e.g. by pasteurization or sterilization) the containers and prevent them from distorting or possibly cracking (bursting) due to increased internal pressure.
For the latter aspects, it has been suggested of carrying out the heat treatment by microwave application in an environment maintained at a superatmospheric (2-3 atmospheres) steam pressure.
While this teaching is successful to prevent the containers of products being processed from cracking or becoming damaged, it has, nevertheless, the important disadvantage that the outermost layers of the product are further overheated which, due to the action by the microwaves, are already at a higher temperature than the innermost ones.
Moreover, steam could condense over the container surfaces, thereby the impinging microwaves are substantially attenuated and the uneven heating of the product being processed is made worse which was bad already. Condensation problems may also be encountered at the process start over the cold parts of the equipment used to apply the heat treatment, which reflects in the uneveness of heating being further aggravated due to unforeseable substractions of useful heat from the product to have the condensation water vaporized.
The problem underlying this invention is to provide a method for heat stabilizing, on a continuous and commerical scale basis, pre-packaged food products by the use of microwaves, and having such operational features as to fully overcome the above-noted drawbacks affecting the prior art.